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Supply of fresh air and removal of exhaust gases by a gas exchanger

The gas exchange process: A basic part of the cycle of an internal combustion engine is the supply of fresh air and removal of exhaust gases. This is the gas exchange process. Scavenging is the removal of exhaust gases by blowing in fresh air.



Charging is the filling of the engine cylinder with a supply or charge of fresh air ready for compression. With supercharging a large mass of air is supplied to the cylinder by blowing it in under pressure. Older engines were 'naturally aspirated'—taking fresh air only at atmospheric pressure. Modern engines make use of exhaust gas driven turbochargers to supply pressurised fresh air for scavenging and supercharging. Both four-stroke and two-stroke cycle engines may be pressure charged.

On two-stroke diesels an electrically driven auxiliary blower is usually provided because the exhaust gas driven turboblower cannot provide enough air at low engine speeds, and the pressurised air is usually cooled to increase the charge air density.

Exhaust gas turbocharging arrangement

An exhaust gas driven turbocharging arrangement for a slow-speed two-stroke cycle diesel is shown in Figure

A turboblower or turbocharger is an air compressor driven by exhaust gas . The single shaft has an exhaust gas turbine on one end and the air compressor on the other. Suitable casing design and shaft seals ensure that the two gases do not mix. Air is drawn from the machinery space through a filter and then compressed before passing to the scavenge space. The exhaust gas may enter the turbine directly from the engine or from a constant-pressure chamber. Each of the shaft bearings has its own independent lubrication system, and the exhaust gas end of the casing is usually water-cooled.

Turbocharger

Fig: A Turbocharger

Related Information:

  1. Scavenging methods - Cross-flow scavenging, loop scavenging, & uniflow scavenging

  2. A basic part of the cycle of an internal combustion engine is the supply of fresh air and removal of exhaust gases. This is the gas exchange process. Scavenging is the removal of exhaust gases by blowing in fresh air.
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  3. Countermeasures against scavenge space fire

  4. Once a fire is detected the engine should be slowed down, fuel shut off from the affected cylinders and cylinder lubrication increased. All the scavenge drains should be closed.
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Marine diesel engine related other useful articles:
  1. Four stroke cycle diesel engines operational guideline

  2. The four-stroke cycle is completed in four strokes of the piston, or two revolutions of the crankshaft. In order to operate this cycle the engine requires a mechanism to open and close the inlet and exhaust valves
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  3. Two stroke cycle diesel engines operational guideline

  4. The two-stroke cycle is completed in two strokes of the piston or one revolution of the crankshaft. In order to operate this cycle where each event is accomplished in a very short time, the engine requires a number of special arrangements.
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  5. Power measurement for marine diesel engine - The engine indicator

  6. There are two possible measurements of engine power: the indicated power and the shaft power. The indicated power is the power developed within the engine cylinder and can be measured by an engine indicator. The shaft power is the power available at the output shaft of the engine and can be measured using a torsionmeter or with a brake.
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  7. Supply of fresh air and removal of exhaust gases by a gas exchanger

  8. A basic part of the cycle of an internal combustion engine is the supply of fresh air and removal of exhaust gases. This is the gas exchange process. Scavenging is the removal of exhaust gases by blowing in fresh air.
    More .....

  9. The fuel oil system for a diesel engine

  10. The fuel oil system for a diesel engine can be considered in two parts—the fuel supply and the fuel injection systems. Fuel supply deals with the provision of fuel oil suitable for use by the injection system.
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  11. Lubricating oil system for a marine diesel engine - how it works

  12. The lubrication system of an engine provides a supply of lubricating oil to the various moving parts in the engine. Its main function is to enable the formation of a film of oil between the moving parts, which reduces friction and wear. The lubricating oil is also used as a cleaner and in some engines as a coolant.
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  13. Cooling of ships engine - how it works , requirement of fresh water & sea water cooling system

  14. Cooling of engines is achieved by circulating a cooling liquid around internal passages within the engine. The cooling liquid is thus heated up and is in turn cooled by a sea water circulated cooler. Without adequate cooling certain parts of the engine which are exposed to very high temperatures, as a result of burning fuel, would soon fail.
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  15. Starting air system for diesel engine - how it works

  16. Diesel engines are started by supplying compressed air into the cylinders in the appropriate sequence for the required direction. A supply of compressed air is stored in air reservoirs or 'bottles' ready for immediate use. Up to 12 starts are possible with the stored quantity of compressed air.
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  17. Governor-Function of governors controlling speed of marine diesel engine

  18. The principal control device on any engine is the governor. It governs or controls the engine speed at some fixed value while power output changes to meet demand. This is achieved by the governor automatically adjusting the engine fuel pump settings to meet the desired load at the set speed.
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  19. Cylinder relief valve of a marine diesel engine - operational guideline

  20. The cylinder relief valve is designed to relieve pressures in excess of 10% to 20% above normal. The operation of this device indicates a fault in the engine which should be discovered and corrected.
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  21. Explosion relief valve of a marine diesel engine

  22. As a practical safeguard against explosions which occur in a crankcase, explosion relief valves or doors are fitted. These valves serve to relieve excessive crankcase pressures and stop flames being emitted from the crankcase. They must also be self closing to stop the return of atmospheric air to the crankcase.
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  23. Turning gear operational guideline
    The turning gear or turning engine is a reversible electric motor which drives a worm gear which can be connected with the toothed flywheel to turn a large diesel. A slow-speed drive is thus provided to enable positioning of the engine parts for overhaul purposes.
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  24. Couplings, clutches and gearboxes of a marine diesel engine

  25. The principal control device on any engine is the governor. It governs or controls the engine speed at some fixed value while power output changes to meet demand. This is achieved by the governor automatically adjusting the engine fuel pump settings to meet the desired load at the set speed.
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  26. MAN B&W diesel engine - Basic principles and operational guideline

  27. It is one of the MC series introduced in 1982, and has a longer stroke and increased maximum pressure when compared with the earlier L-GF and L-GB designs.
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  28. Crankcase oil mist detector of a marine diesel engine

  29. It is one of the MC series introduced in 1982, and has a longer stroke and increased maximum pressure when compared with the earlier L-GF and L-GB designs.
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  30. Various Heat exchanger for running machinery on board cargo ships

  31. Shell and tube heat exchangers for engine cooling water and lubricating oil cooling have traditionally been circulated with sea water. The sea water is in contact with the inside of the tubes, tube plates and water boxes.
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  32. Guideline for Turbochargers safety and operational requirement

  33. Shell and tube heat exchangers for engine cooling water and lubricating oil cooling have traditionally been circulated with sea water. The sea water is in contact with the inside of the tubes, tube plates and water boxes.
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  34. Function of Piston & piston rings

  35. Piston forms the lower part of the combustion chamber. It seals the cylinder and transmits the gas pressure to the connecting rod. The Piston comprises of two pieces; the crown and the skirt.The crown of a piston is subject to mechanical and thermal stresses.
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